Fence systems that are capable of sensing tamper related activity on the fence wire are well known in the security industry. A wire fence system is one in which one or more wires are held in tension between two end supports. An end support may be any support where the wire of the fence is terminated, such as at a fixed post or another fixed physical structure (for example, a wall). A wire fence may include a number of intermediate supports, but need not do so. Reference to a wire fence, or taut wire fence throughout this specification should be understood to include an electrified wire fence as well as a conventional, non-electric, wire fence, unless specifically stated otherwise.
One means of breaching an electric security fence is to cut the wires of the fence. This kind of breach is easily detected by electrical signals that are interrupted by a cut wire and can then sound an alarm for the particular security zone being breached.
Another method by which an intruder can breach an electric taut wire security fence is by climbing the fence. This can result in the fence wires deflecting downwards and connecting together as a short circuit which can be detected as loss or reduction of electrical signal through a wire.
Yet another method of breaching an electric fence is to deflect the wires of the fence, as can occur by a person climbing onto the wire or otherwise pulling the wires apart to enable an intruder or object to pass through the fence. In such instances the wires may not short or open circuit the signal through the fence.
In both electric and conventional fences (not electrified) cutting, bending or spreading the wires changes the tension in the wires, which can be detected by a sensor.
For example, U.S. Pat. No. 4,829,287 discloses a taut wire intrusion detection system including a plurality of tension sensors which transmit signals to a processor associated with a particular post which analyses the signals, and transmits this information to a central control unit. Alarms indicating the location of a potential intrusion are then activated at the central location.
US Patent Application No. 2008/0106408 discloses a taut wire panel system including tensioned sensor wires, where excessive movement of the wire triggers a switch, causing a signal to be sent to activate an alarm associated with the panel in which the sensor wire is located.
It is generally important for the tension along the wire of a wire fence system to be kept within a prescribed range of tensions for effective operation of the fence under the conditions and purpose for which it is to be used.
If the tension is too loose then a cut or deflected wire may not be sensed as enough of a change in tension. If the tension is too high then it may cause problems with insulators, end post connections and to the fence wire—particularly when there is a drop in temperature and the resulting increase in tension on the wire may increase the level of damage. Further, if the tension is too high then the wire may reach its yield point and no longer maintain tension when the temperature cycles between a low temperature and a high temperature.
Additionally, if the tension is too high or too low, it may be outside the range of ideal operation of a tension sensor used to detect deflection of the wire—which may be either outside of a linear range, or in a significantly reduced resolution range of the sensor. In either of these cases the likelihood of detection of a tamper condition or an intruder will be reduced.
Strain gauges are known for measuring tension in a wire. However, such strain gauges require the person installing or maintaining the wire fence to know the optimal tension range for the fence, and read the gauge to mentally compare the reading to the optimal range.
Further, none of the prior art readily enable an installer or maintenance person to identify a fence wire which is outside of the allowable tension limits without manually testing each wire. Even if the maintenance person had prior knowledge of wires being triggered by false readings (assuming an intrusion is not occurring) from a central station before going out to the fence, they are required to manually match the alarm to the fence wire when in the field.
It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.
Throughout this specification, the word “comprise”, or variations thereof such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.